Steps to reproduce:
<ol>
    <li> Enable auto-fill if not already enabled (autofill is usually enabled by default) - chrome://settings, Advanced, Password and forms, "Enable Autofill to fill out web forms in a single click."
    <li> Type some dummy value in the input field and hit submit.
    <li> Refresh the page.
    <li> Dobule-click on the input field to show the suggestions (or type part of what you just typed to see it).
    <li> Select the suggested autofill choice.
    <li> Surprise...
</ol>

<iframe srcdoc="
    <script>
    var lastInputLength = 0;
    </script>
    <form>
        <input name='autofillfield'
            oninput='
            if (Math.abs(this.value.length - lastInputLength) > 1) {
                /* More than 1 character changed - Assume that the change is caused by autofill. */
                frameElement.remove();
            }
            lastInputLength = this.value.length;
        '>
        <input type=submit>
    </form>
"></iframe>
